Device tree bindings for Ethernet chip connected to TI GPMC

Besides being used to interface with external memory devices, the
General-Purpose Memory Controller can be used to connect Pseudo-SRAM devices
such as ethernet controllers to processors using the TI GPMC as a data bus.

Ethernet controllers connected to TI GPMC are represented as child nodes of
the GPMC controller with an "ethernet" name.

All timing relevant properties as well as generic GPMC child properties are
explained in a separate documents. Please refer to
Documentation/devicetree/bindings/memory-controllers/omap-gpmc.txt

For the properties relevant to the ethernet controller connected to the GPMC
refer to the binding documentation of the device. For example, the documentation
for the SMSC 911x is Documentation/devicetree/bindings/net/smsc,lan9115.yaml

Child nodes need to specify the GPMC bus address width using the "bank-width"
property but is possible that an ethernet controller also has a property to
specify the I/O registers address width. Even when the GPMC has a maximum 16-bit
address width, it supports devices with 32-bit word registers.
For example with an SMSC LAN911x/912x controller connected to the TI GPMC on an
OMAP2+ board, "bank-width = <2>;" and "reg-io-width = <4>;".

Required properties:
- bank-width: 		Address width of the device in bytes. GPMC supports 8-bit
			and 16-bit devices and so must be either 1 or 2 bytes.
- compatible:		Compatible string property for the ethernet child device.
- gpmc,cs-on-ns:	Chip-select assertion time
- gpmc,cs-rd-off-ns:	Chip-select de-assertion time for reads
- gpmc,cs-wr-off-ns:	Chip-select de-assertion time for writes
- gpmc,oe-on-ns:	Output-enable assertion time
- gpmc,oe-off-ns:	Output-enable de-assertion time
- gpmc,we-on-ns:	Write-enable assertion time
- gpmc,we-off-ns:	Write-enable de-assertion time
- gpmc,access-ns:	Start cycle to first data capture (read access)
- gpmc,rd-cycle-ns:	Total read cycle time
- gpmc,wr-cycle-ns:	Total write cycle time
- reg:			Chip-select, base address (relative to chip-select)
			and size of the memory mapped for the device.
			Note that base address will be typically 0 as this
			is the start of the chip-select.

Optional properties:
- gpmc,XXX		Additional GPMC timings and settings parameters. See
			Documentation/devicetree/bindings/memory-controllers/omap-gpmc.txt

Example:

gpmc: gpmc@6e000000 {
	compatible = "ti,omap3430-gpmc";
	ti,hwmods = "gpmc";
	reg = <0x6e000000 0x1000>;
	interrupts = <20>;
	gpmc,num-cs = <8>;
	gpmc,num-waitpins = <4>;
	#address-cells = <2>;
	#size-cells = <1>;

	ranges = <5 0 0x2c000000 0x1000000>;

	ethernet@5,0 {
		compatible = "smsc,lan9221", "smsc,lan9115";
		reg = <5 0 0xff>;
		bank-width = <2>;

		gpmc,mux-add-data;
		gpmc,cs-on-ns = <0>;
		gpmc,cs-rd-off-ns = <186>;
		gpmc,cs-wr-off-ns = <186>;
		gpmc,adv-on-ns = <12>;
		gpmc,adv-rd-off-ns = <48>;
		gpmc,adv-wr-off-ns = <48>;
		gpmc,oe-on-ns = <54>;
		gpmc,oe-off-ns = <168>;
		gpmc,we-on-ns = <54>;
		gpmc,we-off-ns = <168>;
		gpmc,rd-cycle-ns = <186>;
		gpmc,wr-cycle-ns = <186>;
		gpmc,access-ns = <114>;
		gpmc,page-burst-access-ns = <6>;
		gpmc,bus-turnaround-ns = <12>;
		gpmc,cycle2cycle-delay-ns = <18>;
		gpmc,wr-data-mux-bus-ns = <90>;
		gpmc,wr-access-ns = <186>;
		gpmc,cycle2cycle-samecsen;
		gpmc,cycle2cycle-diffcsen;

		interrupt-parent = <&gpio6>;
		interrupts = <16>;
		vmmc-supply = <&vddvario>;
		vmmc_aux-supply = <&vdd33a>;
		reg-io-width = <4>;

		smsc,save-mac-address;
	};
};